The present invention relates to a combination seal ring retainer and guide ring on a valve plug for use in a control valve, for example a control valve for a steam turbine in a nuclear power plant.
Control valves, such as steam control valves are known to employ a valve plug which is movable with respect to a valve body or bonnet. Conventional control valves include a hollow cylindrical valve bonnet in which a valve plug is movably disposed. Generally, the valve plug is adapted to move axially within the hollow cylindrical bonnet. Apertures or passages formed through the cylindrical wall of the bonnet can be opened or closed for the passage of fluid depending on the position of the valve plug within the bonnet. For example, the apertures can be closed by positioning the valve plug in a location within the bonnet which blocks the passage of fluid through the apertures. The apertures may be opened by moving the valve plug to a position within the bonnet which does not block the passage of fluid through the apertures. In this manner, the position of the valve plug controls the flow of fluid through the apertures provided in the valve bonnet.
For proper operation, the outside diameter of the valve plug should be sealed against the inside diameter of the valve bonnet. With a proper seal between the valve plug and the valve bonnet, fluid will be inhibited from flowing around or past the valve plug. Thus, fluid flow can be controlled to occur only through the apertures provided in the valve bonnet when the valve plug is moved to a position in which the valve plug does not block the apertures.
Conventional control valve designs have also included a guide ring secured to the valve plug. The guide ring is provided with an outside diameter slightly larger than the outside diameter of the valve plug and operates to guide the valve plug within the valve bonnet and to minimize lateral movement between the valve plug and the bonnet. Since the guide ring has an outside diameter slightly larger than the outside diameter of the valve plug, the valve ring may come into contact with the inside surface of the valve bonnet during operation. For this reason, the guide ring, or at least a contacting surface of the guide ring, is generally made of a harder material than the material from which the valve plug is made.
FIGS. 1--3 show an example of a conventional valve body 1 and valve bonnet 2. The valve body 1 defines a hollow area 3. Apertures 4 in the valve bonnet 2 provide a communication path between the hollow area 3 and a fluid outlet 5. Apertures 6 provide a communication path between a space 7 and the outlet 5 through the surface 8 of a valve plug 10. The valve plug 10 and a guide ring 11 are arranged within a hollow cylindrical portion 12 of the valve bonnet 2 arrangement.
FIG. 2 represents a cross-sectional view of the hollow cylindrical portion 12 of the valve bonnet 2 and a cylindrical or cup-shaped valve plug 10. The valve plug 10 is located within the bonnet portion 12 and is movable in the axial direction of the bonnet portion 12. A valve stem 13 extends from the valve plug 10 in the axial direction of the bonnet portion 12. The stem 13 may be attached to a hydraulic actuator (not shown) which controls the movement of the stem 13 and valve plug 10 in a well-known manner. Fluid (e.g., steam) enters the area 3 in the valve body 1 through an inlet not illustrated, and after passing through the opened position of the apertures 4 exits the valve through outlet 5. In order to prevent excessive pressure unbalance forces on the plug 10, pressure balance holes 6 may be located in the flat surface 8 at the bottom of the plug.
FIG. 3 shows a view of the apparatus shown in FIG. 2 taken along reference line 3--3 of FIG. 2.
As shown in FIGS. 1-3, a guide ring 11 is attached to the rim of the cup-shaped valve plug 10. The guide ring 11 has an outer circumferential surface 14 which extends radially outward farther than the outer circumferential surface of the valve plug 10. The surface 14 of the guide ring 11 is arranged adjacent the inside surface of the bonnet portion 12 (or a liner, not shown, provided on the inside surface of the bonnet) when the valve plug 10 and ring 11 are disposed within the bonnet portion 12 as shown in FIGS. 2 and 3. In this manner, the guide ring 11 assists in guiding the valve plug 10 when the valve plug 10 is moved in the axial direction of the bonnet portion 12.
In this regard, the surface 14 or the entire guide ring 11 may be made of a harder material than the material from which the plug wall 10 of the valve plug is made. Furthermore, it is beneficial to provide the guide ring 11 as a separate element so that the guide ring 11 can be easily replaced without replacing the valve plug.
FIG. 2 also shows two seal rings 15 and 16, such as pressure seal rings, which are disposed about the circumference of the valve plug 10. The seal rings may be provided for inhibiting the passage of fluid (e.g., steam) from the space 3 through the annular gap between bonnet portion 12 and valve plug 10 or ring 11, into the space 7, through the pressure balance holes 6 and to the outlet 5. As will be described below, conventional valve plug designs include ring grooves formed in the outer circumferential wall of the valve plug for holding seal rings therein.
Referring to FIG. 1, a portion of the wall of the valve plug 10 is shown as being secured with a guide ring 11 by a bolt 17. The plug wall 10 has a surface 18 which faces the inside surface of the hollow valve bonnet portion 12.
As shown in FIG. 1, the surface 18 of the valve plug 10 is provided with two grooves 19 and 20 adapted to hold the pressure seal rings 16 and 17, respectively. In order to install the seal rings in the grooves 19 and 20, the seal rings must be stretched so as to fit over the surface 18 of the valve plug 10. While being stretched to fit over the surface 18, the seal rings must be positioned over the grooves 19 and 20, whereupon the seal rings can contract to fit within the grooves 19 and 20. Thus, in order to install the seal rings on a conventional system, the seal rings must be elastically expanded to fit over the valve plug and the guide ring. Such stretching or expansion of the seal rings can cause detrimental permanent deformation of the seal rings and can substantially shorten the life expectancy of the seal rings. Furthermore, the requirement that the seal rings be expandable severely limits the materials from which the seals can be made; that is, the seal rings must be made of a material which allows elastic expansion at the expense of other desirable characteristics.